The invention relates to a magnet arrangement with a magnet coil system for generating a magnetic field in the direction of a z axis in a working volume arranged on the z axis around z=0 with two coil systems that are each arranged in a container positioned around the z axis, wherein the containers are mechanically axially separated from each other by a split, wherein the coil systems each include one first coil section system and at least one of the coil systems contains a second coil section system, wherein each coil section system comprises at least one coil body and at least one winding package, wherein the first coil section systems are exposed to magnetic forces acting in the axial direction toward the split, and wherein the second coil section systems are exposed to magnetic forces acting in the axial direction away from the split, wherein these magnetic forces are at least partly mechanically transmitted to the containers, and with at least one mechanical structure that withstands compressive loads, that is arranged in the split, and which supports at least part of the attractive magnetic forces.
Such an arrangement is known from [1].
The uses of such magnet arrangements include various applications. In experiments to study material properties in a magnetic field using neutrons and X-rays, a small split (typically less than 5 cm at the magnet center) between the two coil systems permits direct access to the sample for the neutron or X-ray beam. The sample is positioned in the working volume through a bore along the axis of the magnet arrangement and is irradiated by the neutron or X-ray beam through the split. The neutron or X-ray beam is detected at outside the magnet system after scattering with the sample. To ensure that as many neutrons or photons as possible arrive at the sample and reach the detectors, the size of the mechanical structure that supports the magnetic forces in the split between the coil systems, in particular, in the region of the neutron or X-ray beams, must be minimized and be made of suitable material. In magnets for neutron scattering, thin aluminum rings are often used in the split because aluminum is very transparent to neutrons (typically 3-4% loss per 1 cm of aluminum). It is also known that the structure is formed in the split as wedges or columns. The free space in the split can also be evacuated during operation to prevent unwanted scattering between the neutron or X-ray beam and liquids or gas.
From [2], a magnet arrangement is known whose magnet coil system includes two different superconducting coil systems that are positioned in two vacuum-tight containers filled with liquid helium and axially separated mechanically by a split. The split is 10 mm at the magnet center and increases radially with an aperture angle of +10° and −50 to increase the aperture cross-section for the beam. This magnet arrangement consists exclusively of coils that are exposed to magnetic forces attracting in the axial direction toward the split. This also applies to all other previously implemented magnet arrangements for neutron and X-ray scattering. Each coil is protected with a separate protection element (low resistance) against overheating and high voltages in case of collapse of the superconductivity (quench) during operation. To produce the vertical magnetic field of 10 Tesla efficiently, the containers must have the thinnest walls possible at the split (3 mm of steel). However, the small distance between the two coil systems causes the attractive axial forces between the two coil systems to be especially strong (approximately 250 kN). In the evacuated split between the containers, thin concentric aluminum rings are therefore inserted that support the attractive axial magnetic forces between the two coil systems in the operating state to prevent deformation of the thin containers. In this mechanical structure, there is no joint or weld between the aluminum and the steel (or other materials), so that no problems arise from thermal expansion when the system is cooled down. The two containers are kept together with three tubes made of steel that are also used for liquid coolants and electrical connections. Sometimes, an additional aluminum ring is screwed in the split to the two containers to keep the two containers together when, in the uncharged state, no magnetic forces are present to compensate the weight forces acting on the containers.
A similar mechanical structure in the split of a split-coil magnet arrangement is also known from [3], wherein this structure consists of C-shaped aluminum rings and spacers.
Such mechanical structures in the split can efficiently support the attractive forces between the two coil systems and prevent deformation of the containers. However, such known devices have the disadvantage that only magnetic forces acting toward the split can be handled.
In [4] and [5], a magnet arrangement is described that comprises two coil systems separated by a split with two coil section systems each, wherein the first coil section systems are exposed to magnetic forces acting in the axial direction toward the split, and the second coil section systems to magnetic forces acting away from the split in the axial direction. Such systems are also known from [6], [7], [1]. One application of such arrangements is open superconducting magnets that are used to produce a magnetic field (of typically 1 Tesla) as part of a system for magnetic resonance imaging (MRI). Open magnets are intended to permit lateral access to the patient between the coil systems during MRI imaging. The distance between the two coil systems is typically greater than 50 cm. Usually, only a few rods are positioned in the split between the two coil systems to support the attractive forces.
The magnet arrangement from [4] consists of two identical, superconducting coil systems that are positioned in two containers mechanically axially separated by a split and that are actively cooled (without the use of cryogenic liquids). Each coil system includes three coils. During operation, two coils are subject to a magnetic force acting in the axial direction toward the split. The third coils are subject to a magnet force acting in the axial direction away from the split. The resulting force between the two coil systems during operation is attractive (765 kN). The three coils of a coil system are connected by a rigid structure made of steel. The split contains four rods that separate the two coil systems and support the resulting attractive force. Each coil is protected by its own protection element (antiparallel pairs of diodes) against overheating and high voltages in case of quenching. In the arrangement of [6], two axially separated superconducting coil systems are described, wherein each coil system comprises two coil section systems that carry electric currents in opposite directions and that are exposed to magnetic forces in axially opposed directions. The two coil section systems are connected by a rigid structure that supports the magnetic forces and thus keeps the coil section systems together. The two coil systems are screwed together through four bearing rods that support the resulting attractive magnetic force and keep the two coil systems apart.
In the arrangement [7], there are, in addition to the rigid structure within the container, numerous braces that support the attractive and repulsive magnetic forces of the various coil section systems and prevent deformation of the containers.
In the arrangement from [1], there are two bearing connecting structures between the two coil systems. This arrangement also has rigid structures within the container in which the coil systems are arranged that support the attractive and repulsive magnetic forces of the various coil section systems. The resulting force between the two coil systems is attracting and is supported by the connecting structures that withstand compressive loads. Transportation securement rods are also provided on the side radially outside the container that act against the vibrations and torsions of the magnet system.
Such mechanical structures in the split are suitable for open MRI systems, wherein the distance between the two coil systems is large. To ensure sufficient lateral access to the patient, the connecting structure between the two coil systems must be as space-saving as possible. For that reason, in the known arrangements, the various coil section systems are interconnected by a rigid structure in each coil system, so that only the resulting magnetic force between the coil systems has to be supported by the smallest possible number of space-saving connecting structures in the split. However, such known connecting structures have the disadvantage that the rigid structures they require for connection occupy space inside the containers that is also required for liquid coolants. The containers must therefore have correspondingly large dimensions.
It is the object of the present invention to propose a magnet arrangement wherein the magnetic forces can be controlled while permitting a low-cost, simple, space-saving and weight-saving assembly.